The Deep End - Minecraft - Update 44

Defeat the Elder Guardian - 30

Elder Guardians are boss mob which live in the Underwater Temples. Underwater Temple are found underwater in the over-world but are extremely rare, spawning mostly in deep oceans. They can be easy to defeat just getting to them can cause problems.

Elder Guardians can shoot you with a ray attack and are usually surrounded by Guardians which swim faster than players can charge at you.

Elder Guardians can drop:
Prismarine Shards
Prismarine Crystals
Raw Fish
Wet Sponge

Elder Guardian